1. Field of the Invention
This invention relates to inkjet printing apparatus for radiation curable ink. The apparatus includes a rotating drum for supporting a substrate during printing.
2. Description of the Related Art
Inkjet printing has increased in popularity in recent years due to its relatively high speed and excellent image resolution. Moreover, inkjet printing apparatus used in conjunction with a computer provides great flexibility in design and layout of the final image. The increased popularity of inkjet printing and the efficiencies in use have made inkjet printing an affordable alternative to previously known methods of printing.
Inks commonly used in inkjet printers include water-based inks and solvent-based inks. Water-based inks are used with porous substrates or substrates that have a special receptor coating to absorb the water. In general, water-based inks are not satisfactory when used for printing on non-coated, non-porous films.
Solvent-based inks used in inkjet printers are suitable for printing on non-porous films and overcome the problem noted above relating to water-based ink. Unfortunately, many solvent-based inks contain about 90 percent organic solvents by weight. As solvent-based inks dry, the solvent evaporates and may present an environmental hazard. Although environmental systems may be available for reducing the emission of solvents to the atmosphere, such systems are generally considered expensive, especially for the owner of a small print shop.
Furthermore, inkjet printers using either solvent-based inks or water-based inks must dry relatively large quantities of solvent or water before the process is considered complete and the resulting printed product can be conveniently handled. The step of drying the solvents or water by evaporation is relatively time-consuming and can be a rate limiting step for the entire printing process.
In view of the problems noted above, radiation-curable inks have become widely considered in recent years as the ink of choice for printing on a wide variety of non-coated, non-porous substrates. The use of radiation curing enables the ink to quickly dry in xe2x80x9cinstantxe2x80x9d fashion without the need to drive off large quantities of water or solvent. As a result, radiation curable inks can be used in high speed inkjet printers that can achieve production speeds of over 1000 ft2/hr (93 m2/hr.)
However, there is a need in the art to improve certain aspects of inkjet printing using radiation-curable ink. In particular, there is a continuing demand to increase the speed of inkjet printing without adversely affecting the quality of the printed image. Such improvements, if attained, could result in a considerable time savings for the operator as well as reduce the need in some circumstances to purchase additional printers to keep up with business demands.
The present invention is directed toward an inkjet printer having a curing device that is adapted to direct radiation such as ultraviolet (xe2x80x9cUVxe2x80x9d) radiation toward ink on the substrate in a manner that helps to optimize the resolution of the final printed image. The inkjet printer of this invention includes a rotating drum for supporting the substrate during printing. The curing device enables the operator to direct radiation, at the operator""s option, to the ink on the substrate only after the ink has moved with the substrate and the drum through an arc that is at least 360 degrees. In this manner, the ink has sufficient time to spread and level on the substrate such that the resulting image is of high quality.
In more detail, the present invention is directed in one aspect to inkjet printing apparatus that comprises a drum for supporting a substrate. The drum has a central reference axis. The apparatus also includes a motor for moving the drum with the substrate in an arc about the central axis. The apparatus further includes a print head for directing radiation curable ink toward the substrate, and a curing device for directing radiation toward the ink received on the substrate. The curing device is selectively operable to direct radiation toward a certain portion of the ink received on the substrate only after that certain portion has moved with the substrate along an arc about the central axis that is at least 360 degrees.
The present invention is directed in another aspect toward a method of inkjet printing. The method includes the acts of supporting a substrate on a drum and moving the drum in an arc about its central axis. The method also includes the acts of directing radiation curable ink onto the substrate, and determining a desired time interval between the time that the ink is received on the substrate and the time that the ink is cured. The method further includes the act of directing radiation toward the ink on the substrate. The act of directing the radiation toward the substrate includes the act of selectively adjusting the time interval between the time that the ink is received on the substrate and the time that the radiation is received by the ink on the substrate such that at least a portion of the ink does not receive radiation until the substrate with the ink portion has moved with the drum along an arc that is at least 360 degrees.
These and other aspects of the invention are described in more detail below and are illustrated in the accompanying drawings.